There is a great need in communication and other electronic systems for obtaining maximum sensitivity to information signals in the presence of noise. As used herein, the word "noise" is intended to refer to random electrical signals and the word "non-noise" is intended to refer to signals which are not random noise. Non-limiting examples of noise signals are Gaussian noise signals. Nonlimiting examples of non-noise signals are coherent signals, carrier signals and modulated signals.
It is common in the electronic art, particularly the communication art to provide a detection threshold gate at the apparatus input to prevent noise signals from triggering false alarms in the apparatus. Often the threshold is adjusted according to the needs of the user, sometimes being set to a low level (i.e., close to the ambient noise level) when great sensitivity is desired and at other times being set to a higher level (i.e., substantially above the ambient noise level) when blanking a strong source is more important than great sensitivity.
In some communication systems where maximum sensitivity is desired, the threshold is set close to the ambient noise level. That is, the ambient noise level is determined and the threshold level (i.e., the level at which the input gate opens to admit a received signal) is set above the ambient noise level by a small pre-determined amount. This ambient noise level is referred to in the art as the "noise floor". When the amplitude of the input signal exceeds the noise floor by the predetermined amount, the gate "opens" admitting the incoming signal to the receiver (i.e., "enabling" the receiver), and when the input signal is less than the predetermined amount, the gate "closes" thereby blocking the input signal from the receiver. The foregoing arrangement for muting receivers is well known in the art.
Despite the widespread use of detection threshold systems in the prior art, several problems remain:
First, in many instances, manual real time adjustment of the threshold level is not possible. In these situations, individual threshold levels are generally preset at conservative levels and thereafter remains unchanged. One of the difficulties with this approach is that it does not take into account aging of the receiver components or changes in supply voltage or other time dependent effects which may cause the signal appearing at the input signal detector to vary in amplitude. While the receiver may still be capable of amplifying these input signals to useful levels, they now fall below the preset fixed detection threshold and are no longer admitted to the receiver. Thus, as the receiver ages, the detector no longer admits such signals and they are lost. Conversely, a drop in the detection threshold due to receiver aging or a rise in the background noise level will trigger the detector, leading to false alarms, i.e., false signal detection.
Second, the threshold level cannot be set to optimally enable the receiver unless the ambient noise floor is known with some precision. Unfortunately, the noise floor is not constant but varies with time. Unless a means is available for conveniently measuring the noise floor and recalibrating the system, the threshold level must be set sufficiently high so that even under the worst anticipated noise conditions, the probability of false alarm is still acceptable, This results in much more conservative threshold settings than is desired.
Third, in multichannel receivers, the individual channels usually have variations in front-end receiver sensitivity, especially in receivers built to operate at very high frequencies and wide bandwidths. This is illustrated in FIG. 1 wherein undulating curve 12 shows the variation in receiver sensitivity (e.g., input gain) across channels 1-N of a multichannel receiver operating, for example, in the 2.0-4.0 GHz range. Curve 12 also depicts the channel-to-channel variation in the noise floor presented to the detector or receiver. As a consequence of such input sensitivity (or noise floor) variations, the signal levels presented to the threshold detection circuitry vary from channel to channel.
In the prior art it has been customary to assign one fixed noise floor threshold, e.g., line 14 in FIG. 1, for each of the channels irrespective of any input sensitivity (or noise floor) variations over time. Detection threshold 14 exceeds the largest anticipated noise signal from channel 16 by some predetermined amount 18. Noise margin 18 is chosen to provide a predetermined false alarm rate for noise triggering of the channel 16 detector. A result of this prior art approach is that while detection threshold 14 and margin 18 are initially appropriate, the optimum threshold level will change because of receiver aging and changes in the ambient noise levels, so that after a time, the detection threshold is no longer optimum.
Thus, there is an ongoing need to dynamically adjust the detection threshold of receiver channels automatically so as to take into account, for example, aging of receiver input components, and/or channel-to-channel sensitivity and noise floor variations in multichannel receivers with time from channel to channel.
As used herein the word "gain" is intended to include attenuation, the word "receiver" is intended to include any type of apparatus for processing or detecting or measuring or analyzing input signals, the word "signal" or "signals" is intended to include signals of any kind, including but not limited to noise signals, modulated signals, carrier signals and combinations thereof, and the word "uncharacterized" with respect to input signals is intended to refer to signals whose non-noise content is unknown, i.e., which may be either substantially pure noise or noise plus non-noise.